Short Circuit Force Calculation Of Conductor

For short circuit force calculation of rigid bus bar F=const*[(Ik3)^2/a]*0.86*(Lc/L)

and for flexible bus bar F'=0.86* F i.e F'=const*[(Ik3)^2/a]*0.75*(Lc/L)

My questions are-

1. why for flexible busbar F'=0.86*F ?

2. For calculation of static mid point sag we use Fst as the Static tensile force of the conductor. From where I will get the value of Fst for different conductors of different type ?

3. What is the physical significance of static mid point sag of a conductor?